The present invention relates to an optical scanning system and, particularly, to an optical system suitable for use in laser beam printers and laser beam scanners.
Conventionally, in producing a scanning light beam by means of a scanning mirror such as a polygonal mirror and galvanomirror, the unevenness of the scanning pitch due to the incline of the scanning mirror is corrected by placing a cylindrical lens shaped longitudinally in the scanning direction at a position distant from the specified scanning points or surface by the focal length of the lens, so that the beam shift from the regular line to be scanned is alleviated.
This method, however, causes a "curvature of field" that is when the beam is at the central position of the scanning surface, the spot is in sharp focus but at the edges it will have a different distance from the scanning surface at the central portion and end portions of the cylindrical lens. Particularly, as the scanning beam is made to have a smaller spot diameter in order to enhance the resolving power, the focal depth becomes smaller and the variation in the spot diameter on the scanning surface increases. This causes a phenomenon, for example, that when the spot is focused correctly at the center of the scanning surface, it is defocused at the end portions on the same scanning line, resulting disadvantageously in the failure of uniformity of the resolving power.